1. Field of the Invention
The invention here relates to shaft travel devices, such as vertical lift arm devices. More particularly it relates to such devices which are pneumatically actuated.
2. Description of Related Art
Shaft travel devices have been used in industry for a number of years. Such devices are intended to allow workers to safely and easily move heavy or bulky objects. Normally such devices are made up of a cylindrical shaft on which is mounted or into which is incorporated a movable cylinder which traverses along the shaft by pneumatic means. Extending radially outwardly from the cylinder is a elongated arm, which may be jointed, and which terminates in some form of gripping device. The cylinder and arm are normally free to rotate around the shaft for all or some portion of a 360.degree. circle.
One of the most common embodiments of the shaft travel devices is the vertical lift art device, and it is this type of device which will be exemplified herein. In the vertical lift arm device the shaft is vertical and the cylinder lifts or lowers the arm and the object carried by the arm. Objects which a worker could not lift or carry (at least not safely or easily) can thus be handled readily, regardless of the worker's size or strength.
The normal operating range of the device is the circle or portion thereof defined by the length of the extended arm and the degree of permitted rotation around the shaft. (Of course, those devices where the shaft is mounted on a track or movable base permits the range to be extended.) In operation the worker rotates the arm around to the location of a object to be picked up and then activates the power system to raise the cylinder and arm to a position above the object. The arm is then lowered so that the gripping device on the end of the arm can be secured to the object. The arm and object are then raised by the power driven unit and the arm rotated through the desired arc to move the object to the location at which it is to be deposited. Upon reaching that position the arm is lowered to place the object in the desired position. The gripping device is then released, the arm raised and swung back to pick up another object or to be rotated to a "parked" position.
Typical applications of these devices includes moving objects from one conveyer belt to another, moving objects from an incoming conveyer belt and stacking them on pallets or vice versa, moving objects from a conveyer belt to an inspection station and then returning them to the conveyor belt after inspection, and so forth.
The usefulness of these devices is based on the recognition that many types of objects are more easily and safely borne and less susceptible to damage if their weight is handled by the mechanical device, so that all the worker has to do is push the arm carrying the object around in its circle. Pushing the arm requires relatively little strength and places very little strain on the worker, as compared to the worker having to lift the object and carry it. The worker is also benefitted in that he or she can work steadily and easily for extended times without risk of injury or fatigue.
Several problems have arisen with such devices, however. It will be recognized that because of the extension of the arm from the cylinder surrounding the shaft and the concentration of the weight of the object at the end of the arm, a substantial amount of vertical torque is exerted against the cylinder. This causes the cylinder to be twisted so that it is distorted away from coaxial alignment with the shaft. In a pneumatically driven unit, it is air pressure retained within a chamber between the shaft and the cylinder which serves to move the cylinder along the shaft and to hold it in a fixed vertical position while the worker rotates the arm to move the object. That air pressure is retained within the chamber by various sealing devices at the top and bottom of the cylinder. The distortion of the cylinder away from a coaxial alignment with the rod causes these seals to also be distorted, such that on the side where the deflecting cylinder is displaced toward the rod the seal is compressed and on the opposite side where the cylinder is displaced away from the rod, the ability of the seal to retain the internal air pressure is diminished. The reduced ability of the seals to maintain the air pressure reduces the degree of control that the worker has during operation of the equipment and, should the seals fail abruptly, substantial damage to the object and the lifting arm device (and perhaps injury to the worker) can be expected.
In addition to the problem of maintaining adequate seals for the operating air pressure, there is often a problem with rotating the arm and cylinder around the shaft. The deflection and distortion of the cylinder discussed above also tends to put unequal loads on the bearings at the upper and lower ends of the cylinder, thus causing the bearings to be compressed against the shaft at one point and to be pulled away from the shaft on the opposite side. Further, when the device is in its fully elevated or lowered position, an upper or lower internal surface of one of the cylinder's heads commonly rests against the internal piston which is secured to the shaft. If the operator tries to rotate the arm while the unit is in the fully extended or lowered position, there is substantial sliding friction between the inner surface of the cylinder head and the piston surface. This makes the rotation of the arm difficult for the worker and may result in jerky motion which can endanger the object being carried or damage the device itself.
Yet another problem is the proper operation and maintenance of lift arm devices in either unusually dirty or clean environments. In a dirty environment, such as when sacks of dusty materials are being handled, the dirt and dust often work into the cylinder and other moving parts of the apparatus and cause binding, scoring of the shaft or bearings and general deterioration of the equipment. Conversely, if the apparatus is to be used in a very clean environment, such as an aerospace clean room, materials such as lubricating oil must not be emitted from the apparatus into the environment.
It is the object of this invention to provide a vertical lift arm device which is substantially free of the problems regarding retention of operating air pressure and interference with smooth and easy rotation of the device's arm, and which can be readily used in a wide variety of environments, including those which are unusually dirty or clean.